It has long been known that safe and competent maneuvering of complex aircraft, such as helicopters, is enhanced by provision of pilot feel in the control sticks. For instance, it is known in helicopters to induce feel into the longitudinal cyclic pitch control stick (referred to hereinafter as "cyclic stick"), by introducing a force which is opposite to the motion of the stick from a null or trim position, so that the pilot can "feel" the significance of a command which his motion of the stick is inducing. It is also known that the null position may be moved from time to time by "trim" controls, so that when the helicopter is flying in steady state conditions, the cyclic stick will stay in a desired position (both longitudinally and laterally) without the pilot having to retain hold of the stick. When flight conditions change, and the null or trim position is desired to be changed, the pilot can press a trim release button on the top of the stick, reposition the stick until desired attitude of the craft and its rotor surfaces have been attained, and reengage the trim control by disengaging the trim release button. The stick will thereafter retain the new, desired trim position until the trim release is subsequently depressed.
The force augmentation, including the elimination of force at the trim position, is effected in most cases by hydraulic servo actuators, the mechanical output of which is effective in either the forwardd or the reverse direction of stick motion directly on the mechanical linkage associated with the cyclic stick. Thus, as the stick is moved from the trim position, the hydraulic actuator creates a force against that motion to provide a proportional feel of the stick motion to the pilot. The hydraulic servo actuator is controlled by an electrohydraulic servo valve which has two outputs, one relating to each of the directions of motion of the hydraulic servo actuator, each output having a hydraulic pressure which is a function of the magnitude of an input electrical signal, the polarity of the input signal determining the polarity of the differential pressure output from the servo valve. Electro/hydraulic force augmentation systems of this general type are illustrated in commonly owned U.S. Pat. Nos. 3,733,039 and 3,719,336.
One problem with this type of system is that a true null position (zero force for zero electrical signal input to the servo valve) is hard to maintain for long time periods over wide variations of temperature of the hydraulic fluid utilized in the servo valve and the hydraulic servo actuator. In fact, temperature variations can equal or exceed the desired control functions to be provided by such devices. When the null shifts as a result of variations in valve or actuator operation due to temperature, the trim position will readjust automatically to compensate, and the stick will acquire a nonlinear feel which is disruptive to proper maneuvering of the craft. Also, the force required to overcome friction in the force augmentation system will vary with drift, causing variation in the stick response to automatic positioning means (such as an autopilot).
Suggestions for overcoming the drift problem may include one-time manual readjustment of gains and/or biases at some point in the system, which can only partially compensate most of the time and will compensate perfectly only on a random, infrequent basis. A full range differential pressure sensor across the output of the servo valve may provide a signal for comparison with the electric demand signal at the input to the servo valve, such that being closed loop, any null offset errors would be compensated for by the feedback. But, closed loop operation naturally requires a pressure range at least equal to the operative differential pressure which may be provided at the hydraulic servo actuator input; and the accuracy (as a percent of full scale) may provide feedback with errors greater than the errors desired to be overcome, unless the pressure sensor and other components are extremely accurate and therefore expensive.